1. Field of the Invention
Exemplary embodiments of the present invention relate to a transmitter/receiver for a wireless communication system; and, more particularly, to an RF signal transmitter/receiver for a wireless communication system.
2. Description of Related Art
In general, wireless communication systems process data and transmit signals using predetermined frequencies. Such wireless communication systems may be classified into wireless communication systems for providing a voice service, and wireless communication systems for providing a packet data service. Examples of the wireless communication system for providing a package data service may include a wireless Local Area Network (LAN) system, a Wireless Broadband (WiBro) system, a Worldwide Interoperability for Microwave Access (Wimax) system, and so on.
Recently, to meet a demand for increasing large-volume multimedia contents, various techniques at a Media Access Control (MAC) layer and a physical (PRY) layer have been developed and used in wireless communication standards such as wireless LAN, WiBro, Wimax and so on. Techniques commonly used in such wireless communication systems will be described below
The above-described systems use an Orthogonal Frequency Division Multiplexing (OFDM) scheme. Using the OFDM scheme, the systems may use multi-carriers. Therefore, the systems have a transmission speed of several tens of mega bytes in a limited bandwidth. Furthermore, the above-described systems use a multi-antenna technology. The multi-antenna technology is a scheme in which different signals are loaded to and transmitted through a plurality of antennas. Theoretically, the multi-antenna technology may improve a transmission speed in proportion to the number of antennas. That is, when the number of antennas increases two or three times larger than when one antenna is used, the transmission speed is improved two or three times higher.
For example, the wireless LAN system has a transmission speed of 11 Mbps in IEEE 802.11b using a Complementary Code Keying (CCK) scheme. However, a transmission speed of up to 54 Mbps may be supported in IEEE 802.11g/a using the OFDM scheme, and a physical layer data rate of 300 Mbps or more may be supported in IEEE 802.11n using the multi-antenna technology.
Such a comparison was made on the basis of the PHY layer. In the MAC layer, a throughput is defined at a data rate which users actually feel on use. The throughput is calculated by dividing the length of packets, which are successfully transmitted among transmitted packets, by a time required for the transmission. That is, the throughput is calculated by dividing the length of successfully transmitted packets by the time required for transmitting the overall packets. For the time required for transmitting the overall packets, a packet header, a preamble, a packet interval, or a back-off time operates as an overhead. Therefore, when the data rate of the physical layer is 54 Mbps, a throughput of about 25 Mbps is acquired in the MAC layer. Furthermore, although the PHY layer supports a data rate of 300 Mbps in IEEE 802.11n, only a throughput of 60 Mbps or less is acquired in the MAC layer. To prevent such a reduction in throughput caused by the overhead, packet aggregation and block ACK schemes are used in the MAC layer. When these schemes are applied, a throughput of 200 Mbps or more may be supported.
As the above-described technology is used, a terminal in the wireless communication system may use the Internet while in motion. Furthermore, the terminal may use video calls and large-volume multimedia contents.
When designing the wireless communication system, a maximum transmission rate which can be supported and a signal transmission distance are the most important two factors. Therefore, the signal transmission distance should be considered together with the improvement of the maximum transmission rate.
Methods used for improving the signal transmission distance will be described below. There may be several methods for improving the signal transmission distance. To improve the signal transmission distance, a link adaptation scheme in the MAC layer and an improvement method using a protocol are generally used. Furthermore, studies on an improvement method using channel coding of the PHY layer or a digital front end have been actively conducted. More specifically, a method of adjusting a transmission power and a data rate in the MAC layer may be used to extend the signal transmission distance. That is because the amount and transmission speed of data which can be transmitted in the wireless communication system are determined based on a distance between a terminal and a terminal, a base station of the system, or an access point and a channel environment. That is, when a distance between objects performing a wireless communication is short and a signal-to-noise ratio (SNR) is good, data may be transmitted at a high speed using a high data rate. On the other hand, when a distance between objects performing a wireless communication is long or an SNR is bad, data should be transmitted at a low speed using a low data rate. Through such a method, it is possible to extend the distance between objects performing a wireless communication.
In terms of power, when a distance between objects performing a wireless communication is short, low transmission power may be used to receive data. Therefore, the transmission power is reduced to minimize power consumption. On the other hand, when a distance between objects performing a wireless communication is long, high transmission power is used to extend a transmission distance. Studies on such a method and apparatus for controlling transmission power have been steadily conducted.
However, since the improvement of gain using a multi-stage amplifier provided in an RF processing unit is based on analog elements having a non-linear characteristic, there may be a limitation. When a power amplifier having a large gain is unconditionally used to increase transmission efficiency of a transmitted signal, short-distance signals reach a saturation state. Then, the signals are distorted. When a power amplifier having a small gain is used to prevent the saturation state, an SNR decreases to degrade performance.
In an RF module, studies on performance improvement for each element are actively conducted. Since the RF module includes analog elements having a non-linear characteristic, a range of guaranteeing gain linearity is limited. Therefore, the distribution of gain from an input stage to an output stage is important. Furthermore, there are many difficulties such as phase noise and the stability of center frequency. The phase noise is generated while a basic frequency is divided to convert an RF signal into a baseband signal, and vice versa. To solve such problems and improve the performance, various studies have been steadily conducted. Recently, studies on an RF module for transmitting data at a high speed in a limited bandwidth by applying the OFDM have been actively conducted. This is because it has become very difficult to satisfy the design and the required performance in the case of the OFDM using multi-carriers, not a single carrier. That is, as the wireless communication systems use multi-carries instead of a single carrier and multi-antennas instead of a single antenna, the design and verification have become complicated. Therefore, a verification method at a system level and an access in terms of system are needed.
A receiver of a wireless communication system receives an RF signal to demodulate into a baseband signal and converts the demodulated baseband signal into a digital signal. A modem of the receiver compensates for a signal distorted by a channel or an analog element and decodes the signal. A transmitter of the wireless communication system encodes data which is to be transmitted, and a digital-analog converter (DAC) converts the encoded data into an analog signal. Further, an RF unit modulates the converted analog signal into an RF signal and transmits the modulated RF signal. To extend a signal transmission distance in such a transmission/reception process, an access should be made in terms of two factors, that is, transmission power and reception signal noise. Before passing through a DAC, a signal is basically determined by the bit number of the DAC and a voltage used by the system. Therefore, the signal should be determined using a small bit number and a small voltage because of the system price and the limited power. After passing through the DAC, the analog signal is amplified by the RF unit using transmission power amplifiers. At this time, all the power amplifiers have a non-linear characteristic. In a linear interval, the gains of the amplifiers are controlled. Such a linear interval ranges from 20 dB to 25 dB. The linear interval is difficult to increase, and a start point of the linear interval is determined based on the maximum transmission power required by a system. For example, a general transmission power amplifier for wireless LAN receives a signal of −30 dBm to −5 dBm, and outputs a signal of −10 dBm to 15 dBm. When the amplifier is replaced with a high-gain power amplifier, a signal of 10 dBm to 35 dBm may be outputted.
Second, reception signal noise needs to be considered, in order to extend the signal transmission distance. A receiver of a wireless communication system includes a low-noise amplifier (LNA) which amplifies a received signal of which the magnitude is small. The LNA is designed to minimize an effect upon noise unlike a general baseband amplifier. When a large signal is inputted, the LNA operates in a low-gain mode to decrease the magnitude of the signal. When a small signal is inputted, the LNA operates in a high-gain mode to increase the magnitude of the signal. In general, a noise figure is small in the low-gain mode, but large in the high-gain mode. Therefore, since the high-gain mode is a mode in which the LNA operates when an input signal is small, a relatively large noise figure decreases a signal-to-noise ration (SNR) of a received signal, thereby degrading the performance of the LNA. To improve the performance of the LNA operating in the high-gain mode, an LNA having a better noise figure may be provided outside the RF system. Then, the noise figure may be improved. However, if the external LNA is unconditionally used, a signal saturation state occurs in an RF unit when a large signal is inputted.
In short, the method of using the high-gain transmission power amplifier and the gain antenna and the method of using the external LNA may be applied to extend the signal transmission distance. However, when the high-gain power amplifier and the external LNA are used at a short distance, the signal saturation may occur in the receiver side. Otherwise, when they are not used, the signal transmission distance may decrease.